Method of manufacturing cold rolled steel sheets



July .8, 1969 f KAZUQ MATSUDO ET AL 3,453,858

METHOD OF MANUFACTURING COLD ROLLED STEEL SHEETS Filed Dec. 20. 1966 Sheet 1 of 2 Primary ColdRelling Rate O l l I Secondary Cold Rolling R FIG.2

Primary Cold Rolling Rate 0) Secondary Cold Rolling Rate INVENTORS V mnrsuoo, 05pm: muR06 i mvo "/un/o HASH/N070 July 8', 1969 Filed Dec. '20, 19 66 KAZUO MATSUDO ET METHOD OF MANUFACTURING com) ROLLED STEEL SHEETS FIG.3

Sheet i of 2 Primary Cold Rolling Rate Secondary Cold Rolling Rafe Plain: Aniloltopy Y Normal Anisotropy A:

----- Grain Size No.

Equi'Heit INVENTORS rm 2 410 MA m w, w/r/w HUM AND United States Patent US. Cl. 72-365 8 Claims ABSTRACT OF THE DISCLOSURE Cold rolled steel sheets having excellent press-forming severity, that is formability and shapability, are manufactured by utilizing a combination of primary and secondary reduction rates, selected from a chart which represents the grain size, normal plastic anisotropy and planar plastic anisotropy which are obtained by combination of the primary and secondary reduction rates at an arbitrary selected standard annealing condition.

This invention relates to a method of manufacturing cold rolled steel sheets by means of a so called two-stage rolling process wherein primary and secondary reductions are relied upon. It is an object of this invention to reasonably produce cold rolled steel sheet of excellent quality by utilizing reduction rates which are most suitable for the characteristics required for the final products and which is determined dependent upon the thickness of the cold rolled steel sheets.

In shaping cold rolled steel sheets into products having the desired configurations and dimensions, there are many complicated problems which occur during the process steps, and no definite method of solving these problems has yet been established. For example, in these process steps, firstly when the limit of shapability of the steel is exceeded, breakage of the sheets would occur. Secondly, wrinkles and wa-rpings may often Occur due to non-uniform stress strain in directions perpendicular and parallel to the plane of the sheets. Thirdly, as is well known in the art, owing to insuflicient rigidity various defects may occur, such as plastic deformation. In the past, while many methods have been proposed regarding the production of such cold rolled steel sheets, each of them could merely solve some of these problems but no method which could solve all of these problems has yet been developed. This is because mechanical properties which exhibit the above mentioned various defects are contradictory to each other. Stated in another way, the above described facts show that ordinary steel sheets, especially rimmed steel sheets are limited in their attainable mechanical properties even by varying their normal rate of reduction or condition of annealing.

As a result of extensive experiments and investigations of the prior method described above, we have succeeded in obtaining cold rolled steel sheets which are most suitable for use in preparing desired products by using manufacturing conditions of ordinary low carbon rimmed steel but by merely changing the combination of the primary and secondary reduction rates. More particularly, the invention contemplates the use of a two stage cold rolling method which permits the providing of a number of combinations instead of utilizing a single step rolling method. Two stage rolling methods are already known in the art. For example, two step rolling methods have been proposed which are especially suitable 'for preparing steel sheets for tin plate and steel sheets for deep drawing. However, by repeating a number of experiments and by adjusting and analyzing the data obtained, we have found a new and novel process of manufacturing cold rolled steel sheets capable of exhibiting better mechanical properties ofpress shapability and formability than prior steel sheets by a suitable combination of the primary and secondary reduction rates. The novel steel sheets can be prepared by only two stage rolling steps.

More specifically, according to this invention steel sheets are provided with required mechanical properties necessary for their respective applications. Thus, by investigating the relation between the primary and secondary reduction rates mentioned hereinabove it is possible to predetermine the required combination of reduction rates which is most suitable to obtain products having properties suitable for intended applications for a given plate thickness of hot rolled steel plates or conversely to determine the thickness of the hot rolled steel plates which is essential to obtain properties most suitable for intended uses from the thickness of the cold rolled steel sheets to be pressed or the thickness of the products. Further, since the invention can establish most effective method of manufacturing the steel sheets suitable for the intended use of the cold rolled steel sheets to be presed without knowing any special condition of manufacturing steel, hot rolling condition or annealing condition, and moreover since the novel invention does not require the addition'of a small quantity of any special element or elements, it is advantageous in that the cleanliness of the steel is high and the cost of manufacturing is economical. In other words, because the method of this invention can provide various types of products for various fields of applications, it is possible to alleviate loss of the properties which are unavoidable when steel sheets of the same quantity are used for a number of different purposes, thus giving rise to a large profit to the users.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which we believe as our invention, it is believed that the invention will be better understood from the following description taken in connection with the accompanying drawings in Which:

FIG. 1 is an equi-height curve showing the relation between primary and secondary reduction rates and the index of the grain size (ASTM number) of one embodiment of this invention;

(FIG. 2 is a graph of equi-height curves showing the relation between primary and secondary reduction rates and the planar anisotropy;

FIG. 3 is a similar graph of equi-height curves showing the relation of said primary and secondary reduction rates and the normal anisotropy; and

FIG. 4 is a combination of FIGS. 1, 2 and 3 and also represents an equi-thickness curve.

Continuing the description of the invention, as has been pointed out before, shapability and formability are required for steel which are to be pressed. With regard to press formability it is required that such property means deformable ability of the steel sheets without any accompanying breakage in either the external force transmission portion or the portion of the sheet deformed thereby as is well known in the art, of the formability, stretchability can be best represented by Erichsen value whereas the deep drawability by Lankford value. On the other hand, shapability means the ability of maintaining given configurations and dimensions during and often transmitting the external force for deformation. However, analysis regarding these problems have not yet been completed so that they are not systematized as is the forma bility. However, it is Well recognized that poor configuration greatly depends upon the yield point of the steel sheets and that the property of maintaining the formed shape could not be considered by ignoring the formability. For these reasons, it will be clearly understood that it is preferable to determine the press-workability of steel sheets dependent upon Erichsen value, Lankford value and yield point of the material. However, it is possible to directly incorporate said three properties into the manufacturing steps of steel sheets. Further, investigation of these factors has revealed that condition of recrystallization is the essential factor that determines these three characteristics. More specifically, it was found that Erichsen values and yield point are not independent of each other but instead they are closely correlated with the grain size, of crystalline. In other words, by adjusting the grain size to a suitable value it becomes not diflicult to obtain the desired Erichsen value and yield point. Further, it is well known by those skilled in the art that such a crystal feature is expressed at Lankford value. In this regard, normal anisotropy represented as Lankford value has strong directionability as that is usually taken as mean value of Lankford values which are respectively measured at 45 and 90 with respect to rolling direction and express the mean value as F. This is because that in actual press works, it is considered that the materials are deformed as a whole independent of the angle of the cut test piece with respect to rolling direction. However, our experiments revealed that this is not sufficient. More particularly, even with blanks having the same value of 5, it was found that there are considerable differences in their limit of drawing. Such a difference can be considered to be caused by planar anisotropy of the material. While there are still remaining unobvious points in the mechanism of creating strains in different directions during actual deformation, it can be said that its not sufiicient to consider F alone and that the sum of Ar and F should be considered.

Of course r greatly varies with the reduction rate. As already pointed out hereinabove the press-workability is basically determined -by grain size and textures of crystals. in accordance with this invention, although the pressworkability of steel sheets is determined based upon the ASTM number which directly indicates the grain size as well as 7 and Ar which are directly related to the assembled structure of the grains, the method of this invention is carried out according to the most common manufacturing conditions of low carbon rimmed steel without the necessity of relying upon a special treatment in which elements in the steel which have large influences upon these factors are removed or such elements are incorporated. Further no special process is required in hot rolling steps of the material following the slabbing operation. Thus, the invention can be carried out under ordinary conditions of manufacturing steel to be pressed.

As the first step of our experiments, we have prepared a number of combinations of the primary and secondary reduction rates which finally result in the same strip thickness, and the'combinations then subjected to aconstant annealing. Upon close investigations and measurements of these sheets with regard to their grain size (ASTM number), normal anisotropy (7) and planar anisotropy (AF), the following astonishing facts were found. Thus, by merely varying the combination of reduction rates said three properties provide very wide and complicated changes, and it was found that a combination wherein the grain size becomes maximum or minimum at reduction rates which results in the same final sheet thickness, a combination wherein normal anisotropy becomes maximum and a combination wherein planar anisotropy becomes maximum or minimum exists. Further, it was found that no combination exists wherein all of these factors could be satisfied, and that the variety of the change is quite different from the change which has been recognized in a single reduction process.

Then we have varied the thickness of the 'hot rolled steel strip, prepared a number of combinations which result in the same final thickness as in the various experiments, rolled and annealed the respective combinations of strips. Upon investigation different results from those of the previous experiments were obtained. This fact shows that even with the same final thickness the values of these properties vary dependent upon the guage of the hot rolled steel strips. Thus it can be said that even when hot rolled strips of different thickness are used said three properties would have identical properties so long as the primary and secondary reduction rates are the same. Throughout those experiments, as the standard annealing condition, both intermediate annealing subsequent to the primary rolling and the final annealing subsequent to the secondary rolling were made at 700 C. for 5 hours.

There is no particular reason for the selection of said temperature and annealing period but they were selected for the simple reason that they are analogous to the actual conditions of the production in actual factories and that the mechanical properties of the final product which are obtained when the temperature and the annealing time are varied independently can .be readily forecast beforehand. Under these conditions, we have continued the experiments to investigate the variation in said three prop erties when the thickness of cold rolled strip was varied or when the thickness of hot rolled strip was varied. The data obtained from these experiments may be tabulated to reduce the investigation into practice. However, in order to use them more conveniently, these data were adjusted and are shown in FIGS. 1 to 3 as equi-height curves. FIG. 4 is a graph to represent a combination of FIGS. 1 to 3.

Hot rolled steel strip utilized in these experiments were prepared from the same charge under the following manufacturing conditions.

Rimmed steel prepared by a LD converter.

Ordinary slabbing mill operation.

Combinations of hot rolling-Finishing temperature: 870

C.; coiling temperature: 575 C.

Check analysis: C, 0.05; Mn, 0.25; S, 0.026; P, 0.015;

For the convenience of understanding, equi-height thickness curves were incorporated into the graph shown in FIG. 4 which represents the result obtained from these hot rolled steel strips. Thus, the manner in which said three properties vary owing to different combinations of the primary and secondary reduction rates for steel strips of the same thickness can be clearly noted. A curve represents an equi-gauge for producing cold rolled steel strip of 0.8 mm. thick by employing a hot rolled steel strip of 5 mm. thick, which also represents the equi-gauge curve for producing cold rolled steel strip of 0.64 mm.

thick from a hot rolled steel strip of 4 mm. thick. The relation between the strip thickness of hot rolled steel and the final strip thickness of cold rolled steel with respect to respective equi-gauge curves and are shown in Table 1 below.

When the curve is'regarded as the equi-gauge curve for a case wherein a hot rolled steel strip having a thickness of 5 mm. is rolled to obtain a cold rolled steel stripof a thicknessof 0.8 mm., combinations of reduction rates which exhibit the maximum and minimum values of said three properties are shown in the following Table 2.

This table shows that even when cold rolled steel sheets having the same thickness are prepared from the hot rolled steel plates under the same manufacturing conditions,',the mechanical properties of the sheets are varied greatly by changing combinations of the reduction rates, and that in ordinary rimmed steel, it is almost impossible to simultaneously satisfy the press-shapability and'the press-formabilitytand shapability with a single reduction rate. However, this fact indicates that it is easy to lmanu- From these three tables it will be noted that when the temperature and period of annealing were variedthe normal anisotropy Iand the grain size vary according to the tendency of variation of these parameters under the standard conditions, whereas the planar anisotropy does not vary substantially. This fact indicates that the trend of the curves under standard conditions are preserved without any appreciable change and that the same time it becomes possible to previously assume the desired values of fipal mechanical properties in ordinary productions. Accordingly,- it is possible to set with great combinations of. reduction rates for producingsteel sheets, which are most suitable for the end use of users without causing any loss of the material as in the case of cut and try methods. 1

.Steel sheets for. productsv such as roofs and fenders of motor cars which require high degree of shapability may' be produced by employing a primary reduction rate of approximately 75% and decarburizing annealing treatment., For ultra-deep drawing a primary reduction rate of above 65% was found suitable. Furthenwhere rigidity after press-forming is especially required as for often plateeofrefrigerator cabinets, a primary reduction rate of about 35% was found suitable. We have also made similar experiments on other changes having somewhat different chemical compositions and succeeded to obtain facture steel sheets having properties most suitable for the requirements involved in the use of the sheets by the user. The detail of one example of the experiments from which FIG. 4 was plotted as follows: Thus, an example of:,the values of said three properties when a hotrolled steel strip of 5 mm. was cold rolled to obtain a sheet of 0.8, mm. thick (curve in FIG. 4) is shown in Table 3 below.

steel sheets having characteristic data substantially identical to those described above.

Curve in FIG. 4 represents an equi-gauge curve of cold rolled steel strip of 0.8 mm. thick obtained from hot rolled steel strip of 0.4 mm. thick. Again the curve -shows a large variation comparable with the curve which is obtained by suitable combination of reduction ratesflsi'milar results can also be obtained for other final sheet thickness, as can be noted from a curve i for the final sheet thickness of 1.2 mm. produced TABLE 3 Primary Grain Mean Planar reduction .size normal anisotrate, ASTM rop percent number ropy r Ar These were obtained under the standard annealing condition or at 7003C. for 5 hours (both for intermediate and final annealing treatments) which were indicated be fore. By treating these strips under ordinary decarburizing annealing conditions, namely, K

Intermediately annealing: 700 0., 8 hours; Final annealing: 730 C., 10 hours decarburize data shown in Table 4'were obtained.

' TABLE 4 AS'IM grain size nu b r... M

from hot strip of 4 mm. thick and from acurve for hot strip of 3 mm. While the charge, temperature and period were varied for each sample, identical trend to that shown in the curve was noted. Thus for example, the grain size and the normal anisotropy shown in FIG. 4 are different, with their numerical values tracing similar curves. This means that in order to provide for the user mechanical properties most suitable for the intended use, it'is firstly necessary to select the thickness of the hot strip which ismost eitective from the view point of the final sheet thickness and that this can be realized very readily.

This constitutes the first feature of this invention and the second feature lies in that it is very easy to find out a combinationf'of reductionrate's from theselected' plate thickness of the hot. strip, which is most suitable for the practice in-the past. Equi-height curves shown in FIG. 4

serve as effective guide for making such a determination and by utilizing this figure it becomes possible to know beforehand the characteristics of .cold rolled steel sheets with great accuracy at the time of actual production. We have actually manufactured cold rolled steel sheets by utilizing curves shown in FIG. 4. The purpose, manufacturing conditions and-mechanicalcharacteristics attained were as follows. i The purpose of the manufacture:

A: to obtain extremely deep drawable material B: to'obtain a material having low yield point r C: contrast material (corresponding to the experiment No. III in the laboratory test) D: to obtain a material having rigidity after press-forming E: control material for the above.

Conditions of steel manufacturing: Rimmed steel was melted in an LD converter.

Same charge was used for above mentioned purposes A, B, C, D and E.

Ordinary slabbing operation as well as hot rolled conditions were used.

Finishing temperature860 C.

Coiling temperature-555 C.

Thicknessmm. for A, B, C and D and 2.3 mm. for E.

RESULT OF THICK ANALYSIS C Mn P S O N Condition of the cold rolling:

Coils A, B, C and D were rolled using the reduction rate shown in curve in FIG. 4, while coil E was rolled by a single rolling operation.

Thickness: 0.8 mm. for respective coils. Reduction rates:

A: Primary 60%; secondary 60% B: Primary 75%; secondary 36% C: Primary 36%; secondary 75% D: Same as C E: 65.2%.

The primary and secondary reduction rates for A, B, C and D respectively represented by cross-points P, Q and R on the curve in FIG. 4. Since E is a control material which was prepared by a single rolling operation, the reduction rate thereof is represented by a crosspoint 0 on the secondary cold reduction line.

Conditions of annealing (open coil annealing for coils A, B and C):

Data of material tests;

A B C D E GraiuslzeASTM No 6.8 6.2 7.2 9.1 9.0 Yield point,kg./mm 14.8 13.7 17.0 22.4 21.9 Tensile strength force, kg./mm 28.6 28.1 29.4 33.0 33.4 Total elongatiompe rcent 55.7 54.0 52.8 48.2 47.3 Normalanisotropyr... 2.09 1.97 1.97 1.43 1.21 Planner anisotropy AIL. 0.63 0.26 0.36 0.32 0.47 Erichsen value,m. mm. 13.1 12.8 11.8 11.1 10.6 V. mm. 36.0 36.5 37.4 38.1

1 Remark: C.C.V. means Fukui's Test Value. 2 Severed when drawn.

Judging from these results of factory tests, it will be clearly understood that the selected reduction rates selected from FIG. 4 depending upon the end use of steel sheets are correct. The fact that the coil A rolled with a primary reduction rate of 60% is a material having mechanical properties most suitable for ultra-deep drawing operation can be evidenced from its values of F, Br and C.C.V. Of course, coils B and C are also suitable for deep drawing operation, their values of F, Er and C.C.V. are not so high as those of the coil A, as clearly indicated by the above table illustrating test data of their mechanical properties. Where the end use of the user especially requires the ability of deep drawing, most satisfactory results could be obtained from the same material by utilizing primary reduction rates of approximately from 60 to 65%. This can be readily understood from the data regarding the coil A and that of the sample II of the laboratory experiment. As can be clearly noted from FIG. 4 identical trends appear not only when the material is prepared by reducing a hot rolled steel strip of 5 mm. thickness but also as well by reducing plates having different thickness. As a result where deep drawability is desired it is preferred to use a primary reduction rate of about to independent of the thickness of the hot strip. Similarly where shapability of the product is of prime importance, the reduction rate for the coil B should be used. According to the test results of mechanical properties YP, YP/ TS and Ar are lower than those of coils A and C.

Although coils A and C have satisfactory mechanical properties which can assure good shapability as in the case of drawability, it is evident that a reduction rate of the order for the coil B results in excellent result in order to impart the maximum shapability. FIG. 4 shows that this is not limited for only the hot rolled steel strip having a thickness of 5 mm., but rolled hot steel strip having any other thickness show the same trends. We prefer to adopt a primary reduction rate of at least more than for the reduction rate most suitable for exhibiting best press-shapability independent of the thickness of hot rolled steel strip. Further it was found that the same reduction rate as that of coil C or D was most suitable for the field of application which require press shapability, especially rigidity. In the case, as can be noted from the above table, which indicates the result of mechanical characteristics, decarburization treatment is not required. The results of mechanical tests of the coil D was more excellent than those of the coil E which was prepared by a single rolling operation, and said results were far larger than those initially expected by us. The fact that, notwithstanding very small grain size, that is 9.1 of the coil D, the value of Er is high, that is 11.1 and the values of F and C.C.V. are 1.43 and 37.4 respectively, shows that the sheet of the coil D provides a very good deep drawing property in addition to rigidity. Further, even applications where rigidity after press-formability is required substantially good press formability is necessary, and such properties are far better than that of the coil E shown in the above described table of the results of the mechanical test.

We have assumed that the secondary reduction rate of more than at least 70% should be used as a result of investigation of the equi-height curves shown in FIG. 4 which are plotted from the results of laboratory experiments for steel sheets which are required to have said rigidity and drawability. In order to confirm this assumption, We have made the following factory experiments.

Coil F: Up to the slab, the same factory experiment as mentioned above was used. Hot rolling conditions:

Annealing condition and tempering condition:

Same as those of coil D.

Position of sampling:

Same as in factory experiment.

Data of the thickness analysis for coil F 9 Data of mechanical test ASTM grain size number of F 9.0 Yielding point kg./mm. of F 21.5 Tensile strength kg./mm. of F 32.5 Total elongation percent of F 48.7 Normal anisotropy r of F 1.40 Planar anisotropy Ar of F 0.42 Erichsenvalue mm. of F 11.0 C.C.V. mm. of F 37.3

The above described mechanical properties were identical to those expected. More specifically, in spite of very small grain sizes, values of Er and C.C.V. were satisfactory and similar to those of coil D and values of r and Ar are better than those of the coil E. According to the data of coil D and E, we prefer to select the thickness of hot rolled steel strip so as to produce sheets of the final thickness at a primary reduction rate of more than 40% and at a secondary reduction rate of more than 70% in order to simultaneously impart to the product the required press-formability and rigidity after press-forming.

Although the above description refers to an example of factory experiments utilizing curves shown in FIG. 4, it will be understood that it is possible to utilize any particular material suitable for respective applications without relying upon any special method of manufacturing but by merely suitably selecting a particular one of the curves shown in FIG. 4. Further, it should be understood that if annealing conditions conventionally utilized in the manufacture of steel sheets were utilized the equiheight curves shown in FIG. 4 would maintain the same trend although their values are required.

As described hereinabove the invention provides a method of manufacturing cold rolled steel sheets which can preserve their mechanical characteristics suitable for their end purpose by a two step cold rolling method. Thus, the investigation makes it possible to use most effectively the same material and greatly benefits both the maker as well as the user of such cold rolled steel sheets.

While the invention has been explained by describing particular embodiments thereof, it will be apparent that improvements and modifications may be made without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. A method of manufacturing cold rolled steel sheets, comprising the steps of cold rolling sheet steel material by a two step rolling operation having a combination of primary and secondary reduction rates, which imparts to the final cold rolled sheet grain size, normal anisotropy, and planar anisotropy resulting from the combination of primary and secondary cold rolling reduction rates, and annealing at a standard annealing condition selected arbitrarily.

2. A method of manufacturing cold rolled steel sheets by a two step rolling operation as recited in claim 1 rolling low carbon rimmed steel plate with a secondary reduction rate of more than to achieve maximum rigidity after press forming.

3. A method of manufacturing cold rolled steel sheets by two step cold rolling procedure as recited in claim 1 and further comprising the steps of cold rolling rimmed low carbon steel plates at a primary reduction rate of about from 55 to 70%, and then subjecting the plates to a secondary cold rolling at a secondary reduction rate selected to obtain steel sheets of predetermined final thickness, the combination of said primary and secondary reduction rates being selected such that the resulted cold rolled sheets will have excellent press-formability of normal anisotropy.

4. A method of manufacturing cold rolled steel sheets by two step cold rolling procedure as recited in claim 1 and further comprising the steps of coldrolling rimmed low carbon steel plates at a primary cold reduction rate of at least more than 70% and then subjecting the plates to a secondary cold rolling at any secondary reduction rate to obtain the steel sheets of the final thickness, the combination of said primary and secondary reduction rates being selected to provide cold rolled sheets having the maximum shapability and the maximum planar anisotropy.

5. A method of manufacturing cold rolled steel sheets consisting primarily of rolling the steel sheets initially at a preselected primary reduction and then rolling the sheets according to a preselected secondary reduction rate, both of said rates together reducing the sheets from an initial thickness to a predetermined final thickness thereof, said primary and secondary reduction rates being selected according to the desired grain size, normal anisotropy, and planar anisotropy.

6. A method as recited in claim 1 and wherein the steel sheet is initially prepared according to any conventional method before being subjected to the two step rolling operation.

7. A method as recited in claim 1 and wherein the steel sheet has initially been hot rolled before being subjected to the two step rolling operation.

8. A method as recited in claim 2 and wherein the primary reduction rate is substantially less than said secondary reduction rate.

References Cited UNITED STATES PATENTS 2,325,190 7/1943 McElhinney 72-33 3,130,614 4/1964 Nachtman et a1. 72--365 3,309,906 3/1967 Bernick et al. 72-365 3,332,263 7/ 1967 Beadle et a1. 72-7 CHARLES W. LANHAM, Primary Examiner. LOWELL A. LARSON, Assistant Examiner.

. U.S. Cl. X.R.. 72--34 

